Liquid phase epitaxy (LPE) has been mostly used as a technique of crystal growth for mass production of semiconductor laser devices. Lately, the conversion from LPE to MO-CVD (Metal Organic Chemical Vapor Deposition) which has the capability of precision control of layer thickness and is suitable for mass production has hastened in view of enhancement in efficiency and reduction in cost.
FIG. 4 shows a cross-sectional view of a prior art SBA (Self-Aligned Bent Active layer) laser which has a laser structure suitable for production by MO-CVD and which is disclosed in Japanese Laid-open Patent Publication No. 60-192380. In FIG. 4, reference numeral 1 designates a p-type GaAs substrate. A first p-type AlGaAs cladding layer 2 is provided on the p-type GaAs substrate 1. An n-type GaAs current blocking layer 3 is provided on the first p-type AlGaAs cladding layer 2. A second p-type AlGaAs cladding layer 4 is provided on the current blocking layer 3. A p-type or n-type AlGaAs active layer 5 is provided on the second p-type AlGaAs cladding layer 4. An n-type AlGaAs cladding layer 6 is provided on the active layer 5. An n-type GaAs contact layer 7 is provided on the n-type AlGaAs cladding layer 6. An n-side electrode 8 is provided on the contact layer 7. A p-side electrode 9 is provided on the rear surface of the substrate 1. A stripe groove 10 is provided in the current blocking layer 3.
In this SBA laser structure, the bottom width of stripe groove 10 is above 2 .mu.m, and the thickness of second p-type AlGaAs cladding layer 4 is about 0.2 .mu.m.
This laser structure can be produced in two Mo-CVD crystal growth of steps. In the first crystal growth step, the first p-type AlGaAs cladding layer 2 and n-type GaAs current blocking layer 3 are produced on the p-type GaAs substrate 1. After the first crystal growth step, the stripe groove 10 is produced by etching. In the second crystal growth step, the second p type AlGaAs cladding layer 4, active layer 5, and n type AlGaAs cladding layer 6 are successively grown on the wafer, thereby constituting a double-hetero-junction structure. In the second crystal growth step by MO-CVD, the active layer 5 is grown to have a bent configuration like the step-like configuration of the stripe groove 10.
The device operates as follows.
When a voltage is applied to this laser with making the side of p-side electrode 9 plus, current flows concentratedly through the aperture portion of the stripe groove 10. Then, holes are injected into the flat portion 11 of the active layer 5 near the bottom of the stripe groove 10 from the side of second p-type AlGaAs cladding layer 4 and electrons are injected thereinto from the side of n-type AlGaAs cladding layer 6, and they recombine to produce light emission. When current is increased above the threshold current, the device starts laser ocsillation. The active layer of this SBA has a bent configuration similar to stripe groove 10, and at a plane including active layer 5 at the bottom area of the stripe groove 10, the effective refractive index changes in the horizontal direction. Accordingly, laser oscillation occurs at the flat portion 11 of the bent active layer 5, which portion becomes an active region.
In the prior art SBA type semiconductor laser device of such a construction, the bottom width of stripe groove is as large as 2 .mu.m or more and the distance from the active region to bottom of the groove is small as about 0.2 .mu.m. Such a configuration means produces a wide active region which makes transverse mode laser oscillation unstable. The small distance from the active region to bottom of the groove of about 0.2 .mu.m means that a regrowth interface containing a high density of lattice defects and dislocations is located near the active layer. This regrowth interface location results in the likelihood that the device may deteriorate because of these lattice defects in a long-term operation.